Currently much effort is directed toward differentiating embryonic stem (ES) cells to specific lineages in culture for the clinical purpose of providing a source of somatic cells for transplants. To take advantage of the full potential of stem cells, both adult and embryonic, the intrinsic/extrinsic controls that maintain stem cells as stem cells and the extrinsic controls that can then push them along a specific lineage must be realized. With this information, stem cells may be guided to prescribed fates in order to function in replacement therapies for devastating diseases, injury, or birth defects. ES cells are capable of forming all cell lineages, but are transient, while adult stem cells appear to reside permanently in most tissues, including brain, bone marrow, liver, skin and muscle, and are thought to have a more limited capacity for differentiation. Neural stem cells (NSCs) from the adult mouse brain were recently shown to be multipotential, capable of forming most cell lineages, if placed in the appropriate in vivo environment. However, little is known concerning the in vitro differentiation capacity of NSCs. To test the hypothesis that embryonic, adult, and aging NSCs are equivalent to ES cells in their capacity for differentiation, we have 3 aims: (1) to compare NSC and ES cell differentiation along a neuronal lineage to dopaminergic neurons, (2) to compare NSC and ES cell differentiation along a neuro/endocrine pathway to pancreatic islets, and (3) to assess the functionality of differentiated cells following transplantation into syngenic or immunodeficient hosts. The goal of this proposal is to gather initial data for a comparative study of mouse neural stem cells (NSCs) and mouse embryonic stem (ES) cells in their capacity to differentiate along prescribed in vitro pathways and then to function in vivo. This data is crucial for our understanding of stem cell biology and its potential therapeutic use.